There are many different properties which are desirable in automotive tires, and the designers and builders of tires utilize many different rubber blends for different tire components, such as treads, side walls, etc. in an attempt to optimize the various properties of the tire. One of the components of a tire is the tire tread component and this component itself requires many different properties which designers attempt to optimize. For example, lower energy consumption of vehicles is a continuing goal and one factor in the energy consumption of vehicles is the rolling resistance of the tire which is a function primarily of the tire tread composition. Additionally, it is desirable to have a high wear resistance of the tread to avoid early or premature wear of the tire which takes place due to normal operating and certain abnormal conditions. Additionally, it is not only desirable but essential to have a reasonably good traction of the tire so that it provides the necessary skid resistance to the vehicle. Moreover it is also desirable to have a relatively low T.sub.g (i.e. glass transition temperature) of the tire in order that the tires will not fail in extremely cold weather. Moreover, various ingredients of the rubber compositions while improving one property may adversely affect one or more of the other properties. Thus, a primary goal of the tire designers and makers is to provide a rubber composition for the tread component of the tire which seeks to optimize these various properties by balancing the selection of relative amounts of these ingredients without causing any one of desired properties to be unduly unfavorable.
In the blending of the compositions for use as a tire tread component, it is conventional to blend natural rubber with different synthetic rubbers formed by various polymerization processes. Some of the original work done in this field provided polymer blends of various butadiene rubbers with emulsion SBR. Emulsion polymerization was developed extensively and there is extensive use of emulsion polymerization because of certain technological advantages as well as economic advantages in using existing equipment. However, emulsion polymerization has some draw backs and limitations as to the type of products it can produce. Specifically, emulsion polymerization generally is not capable of clearly controlling the diene microstructure of the produced rubbers such as the percent of vinyl in the polymerized product. This in turn has posed limitations as to the properties which could be achieved since various microstructures such as the amount of vinyl groups and other microstructure configurations affect certain properties of the end product.
These draw backs, which are limitations in emulsion polymerization, were overcome by the development of solution polymerization of synthetic rubbers. In solution polymerization there is the possibility of using different catalysts and modifiers, different solvents, different temperatures and varying other types of ingredients and conditions to more closely control the resulting microstructure over a much broader range of polydiene microstructures, especially the ability to control the vinyl content over a much broader range than with emulsion polymerization which generally is limited to the range of about 18-24% vinyl. However, solution polymerization requires new facilities and cannot be accomplished in those facilities which are constructed and utilized for emulsion polymerization without extensive reconstruction. Further, solution polymerization requires the use of organic solvents and expensive materials as well as requiring complex recovery systems for the waste products. Therefore, it would be desirable to provide synthetic rubbers which would provide compounds for tire treads which can be made by emulsion polymerization and which would have improved properties matching those achieved with synthetic rubbers produced by solution polymerization.